As medicine continues to progress, new advanced medical imaging technologies are being developed. With applications ranging from diagnostic procedures to radiation therapy, the importance of high-performance medical imaging is immeasurable. Some high-performance medical imaging systems allow dynamic imaging (i.e., imaging of moving events or scenes). However, these imaging systems must be able to image at high frame rates (i.e., ≧30 frames/second). Some such imaging systems utilize amorphous silicon flat panel x-ray detectors.
Generally, in amorphous silicon flat panel x-ray detectors, an amorphous silicon array is disposed on a glass substrate, and a scintillator is disposed over the amorphous silicon array. The scintillator converts x-ray photons to visible light, and then the amorphous silicon array converts the light into electrical charge. The charge at each pixel on the amorphous silicon array is then read out digitally by low-noise electronics, and is sent to an image processor. Thereafter, the image is displayed on a display, and may also be stored in memory for later retrieval.
The amorphous silicon array comprises field effect transistors (FETs) and photodiodes, typically arranged in rows and columns, wherein the FETs act as switches to control the charging of the photodiodes. The source of each FET is connected to a photodiode, and the drain of each FET is connected to readout electronics via data lines or contact leads.
Current amorphous silicon flat panel x-ray detectors experience certain electrical phenomena that cause imaging difficulties—memory effect and lag effect. Due to the imperfect nature of amorphous silicon FETs, once the FET is turned off, the charge is retained temporarily in the FET, which is known as the memory effect. This transient retained charge bleeds out, or decays, over time, which corrupts the signal being sent to the image processor. Therefore, generally a certain amount of settling time is necessary once a FET is turned off, before signal sampling can occur. As this settling time can take up a significant portion of the total available readout time (i.e., in some cases, it can take up to ⅓ of the total available readout time), reducing or eliminating this settling time will free up more time for signal sampling and/or shorten the line time. Reducing the line time is key to achieving desirable high frame rate imaging (i.e., more than 30 frames/second).
Lag is another undesirable property that exists in current amorphous silicon flat panel displays. The lag effect is caused by residual signals that are left over from the previous image frames, which can cause “ghost images” in imaging techniques such as fluoroscopy. Generally, this lag effect is managed by complex and cumbersome software correction schemes. Therefore, it would be desirable to be able to reduce the lag via a simple and robust means inside the detector hardware instead.
Since existing amorphous silicon flat panel x-ray detectors have imaging limitations, it would be desirable to have amorphous silicon flat panel x-ray detectors that lacked those restrictions. Specifically, it would be desirable to have amorphous silicon flat panel x-ray detectors that utilize sampling methods that shorten readout time and reduce lag therein.